1. Field of the Invention
This invention relates to a scanning probe microscope for obtaining microscopic data on the surface of a specimen by scanning the surface with a probe held close to it and, more particularly, it relates to a barrier height measuring apparatus realized by utilizing a multifunctional scanning type probe microscope capable of obtaining two different sets of data on the surface of a same specimen.
2. Description of the Related Art
There have been known probe microscopes of varied types including the scanning tunneling microscope (STM) and the atomic force microscope (AFM).
The STM is an apparatus proposed by Binnig et. al., in U.S. Pat. No. 4,343,993 for microscopically determining the profiles of objects and reputed for its ability to determine the profiles of conductive specimens by a resolving power of atomic level.
Theoretically, this apparatus is based on the finding that a tunnel electric current flows between a pointed conductive probe and a specimen when the probe is placed very close to the surface of a conductive specimen, for example by 1 nm, and subsequently a bias voltage is applied between the probe and the specimen. The intensity of the tunneling current is expressed by formula (1) as shown below. EQU I.sub.T =B(V.sub.T)exp(-A.phi..sup.1/2 S) (1)
where I.sub.T is the intensity of the tunnel electric current, B(V.sub.T) is a coefficient dependent on the bias voltage applied to the probe and the specimen, A is a numerical coefficient equal to 10.25 nm.sup.-1 (eV).sup.-1/2, .phi. is the barrier height to be determined and S is the distance between the probe and the specimen. Since the barrier height .phi. of any point of a clean surface of a metal is found between 1 and 5 eV, it may be seen from the formula (1) that the intensity of the tunnel electric current can be varied by a magnitude of tens when the distance S between the probe and the specimen is varied by 0.1 nm. The probe of a STM is moved primarily horizontally along an xy plane above the specimen by a fine drive device such as a piezoelectric body to raster-scan the surface of the specimen, while it is also moved vertically or in the z direction so that the distance S between the probe and the specimen is kept constant with an accuracy of 0.01 nm to keep the intensity of the tunneling current flowing between them to be accurately constant. Thus, the probe traces an imaginary and mostly irregular surface which is identical with the surface of the specimen but separated from the latter by a given distance. Then, an STM image showing fine irregularities of the surface of the specimen will be obtained by recording the geometric locus of the tip of the probe along the xy plane and at the same time the voltage applied to the piezoelectric body to move the probe in the z direction and combining them in an appropriate manner. The tunneling current detected by the STM reflects the distance S between the specimen and the probe and at the same time the barrier height .phi. for a specific point of the surface of the specimen which reveals local electronic states of different surface areas of the specimen. Now, it may be appropriate to describe here briefly a barrier height .phi.. The barrier height .phi. used in equation (1) above is defined by formula (2) below. EQU .phi.=(.phi..sub.1 +.phi..sub.2)/2 (2)
where .phi..sub.1 is the ionization potential of the atoms of the material constituting the probe and .phi..sub.2 is that of the atoms of the material constituting the specimen. Since the ionization potential is specific to each material, the material constituting a specific point of the surface of the specimen can be determined from the barrier height .phi. of that point when the material of the probe is known.
A detailed account of a method for determining the barrier height .phi. from the tunneling current detected by an STM is given in Physical Review Letters, Vol. 60, No. 12, 1988, pp. 1166-1169.
The method described in this paper consists in causing the probe to finely vibrate in a direction perpendicular to the surface of the specimen and detecting the distance between the specimen and the probe and at the same time the intensity of the tunneling current when the profile of the specimen is determined by a STM. In this way, the barrier height .phi. which is the metric differential of the intensity of the tunneling current can be obtained for each and every point of the surface of the specimen along with data on the profile of the specimen. While the signals representing the intensity of the tunneling current detected by this method contain oscillatory components, the distance between the specimen and the probe can be so controlled by a feedback control system that the average intensity of the detected tunneling current is always kept constant because the frequency of the fine oscillation is made to be much higher than the cut-off frequency of the feedback control system for controlling the distance between the specimen and the probe. Consequently, the profile of the specimen which is exactly identical with the one acquired by using an ordinary STM can be obtained from the output of the feedback control system.
The above described method for determining the profile of a specimen by utilizing a tunneling current to control the distance between the probe and the specimen and at the same time the barrier height, however, is accompanied by a drawback that they can be determined simultaneously only by means of a bias voltage that should be found within a very limited range. This is because only a limited number of electrons participate in the tunneling current flowing between the probe of the STM and the specimen, the limitation being imposed by the bias voltage, so that the level of the bias voltage by turn should be so controlled as to excite only those electrons that are found on the surface of the specimen if only the atoms on the surface of the specimen are to be effectively detected to determine the profile of the specimen. The use of a bias voltage out of a limited range does not provide any accurate data on the profile of the specimen and therefore it is not possible to determine both the profile of the specimen and the barrier height for each and every point of the surface of the specimen simultaneously if the level of the barrier voltage is in appropriate.
While the proportional relationship between the displacement of the tip of the probe in the z direction and the voltage applied to the fine drive device to move the probe is utilized when the barrier height is determined by using an STM, this technique does not ensure an accurate measurement of the barrier height since the voltage applied to the fine drive device does not necessarily accurately reflect the distance between the probe and the specimen because of the involvement of indeterminable factors in the displacement of the tip of the probe including the non-linearity of the actuator, the elastic constant of the tip of the probe and the interatomic force between the specimen and the probe.
Besides, since the method for simultaneously measuring the profile of a specimen and the barrier height for each and every point of the surface of the specimen as described above utilizes a servomechanism for controlling the distance between the probe and the specimen in order to maintain the tunneling current at a constant level, the servomechanism can bring them very close to each other until they eventually collide against each other in order to keep the tunneling current to a predetermined level if a poorly conductive object exists on the surface of the specimen.